Variable band width amplifier



sept. 1o, 1940. H. RQ'THE'ETAL 2,214,627 VARIABLE BAND WDTH AMPLIFIER Filed June 3, 1937 2 Sheets-Sheet l INVENTO RS HRST ROT/IE WEKA/E yf BY ATTORN EY Sept. 10, 1940. y H R01-HE ET AL 2,214,627

VARIABLE BAND WIDTH AMPLIFIER Fired June s. 19s? 2 sheets-sneer 2 Rg S - y l H 09 T0 TFCTOR OUTPUT INVENTORS HORST ROT/F ATTORN EY Patented Sept. 1Q, 1940 ,meer

VARIABLE` BAND WIDTH vAMPLIFIER v Horst Rothe and Werner Kleen, Berlin, Germany,v .y assignors to Telefunken Gesellschaftfr Drahtylose Telegraphie m. b. H., Berlin, Germany, a y corporation of Germany Application June 3,1937, seriaiNo. 146,133

In Germany June l23, 1936 v aolaims. (01179-17152 The invention relates to an amplier circuit in which the band width is varied by varying the inner resistance of the tube in whose plate circuit the resonant circuit' to be influenced, isinserted.

It is often required that ampliers operating ywith selective frequency, more especially high frequency and intermediate frequency ampliers, be so designed that the degree of selectivity, i. e. l` the bandwidth of 'the admitted frequency range, can be varied bys'imple. means within certain, and often very wide, limits. It would be desirable to maintain, atithis variation in the band width, the degree ofamplification constant at l least to a-cert'ain'degree. This condition isV fullled by the present invention.

A number of circuits are knownv in which the desired degree of amplication of a stage is obtained by varying an operating potential of the Q0 respective tube', for instance the control grid potential, or the screen grid potential; Further more, av control of the/amplification can be accomplished, as is known, by influencing the inner resistance of the tube. If in these arrangements .7.45 a variation in the band width was obtained simultaneously with the control of lthe amplification, this was not the purpose intended by the measures taken, andmore particularly, the control of theamplication has'not` been carried out 3,9 'with' special consideration tothe band width variation."` Itis, a1so,fknown.to control the band width by influencing the inner resistance of ,a pentodewith the 'aid of a Variation of the reutarding vgrid (suppressor grid) potential. The 35 more negative the retarding grid of a pentode is made, the lower becomes the inner resistance. But with this kind of :control it is entirely impossible to maintain the degree of amplification constant during the control performance;4 not F40 even to afair degree. d f Y f In accordance with the invention, a variation of the band width of thev frequency range ad'- mitted in an amplier stage achieved by varyv ing the'innerresistance of theamplier tube in 45 rwhose plate circuit a resonant circuit is inserted. VI.For a better understanding of the invention, I"several points regarding the inner resistance of a tube operating with'` what has been vtermed current distribution ywill Vbe considered. A tube is 'o `said to be operatedwith current distribution" when the cathodey current divides between the yscreen and plate electrodes in accordance ywith .the instantaneous plate potential. For the inner .as ileee @-Qentode Qr ,0f La Sreen f grid tube free of secondary emission', there exists,v as is known, the following equation: l

Jl1 'v 5 whereinV b' represents the constant .of the inner resistance determined bytheconstruction of the electrode system of-the tubeifUpdesignates the plate voltage; andJa is theplate current (see the .journal.y Die Telefunkenrhre' of 'April l0 1936). lThe inner vresistance thus becomes 'the higher, the higher the 4plate potential and the smaller the plate current. The Value of the screen grid potential has no direct influence upon the inner resistance, but only anindirect `effect -l in vthatit is also ya factor whichy determines the plate current. Upon an increase inthe plate current, the inner resistance of the tube becomes lower, and theresonant circuit in the Aplatecircuit;is subjected to an ever'increasing damping 20 whereby ,the degree of thedamping depends on the value ofthe plateY potential. In considering these functions the inner resistance of the tube is to be assumed in parallel vto thel resonant circuit. By Varying the plate current,A or theplate potentiaLv or both, the selectivity of the resonant circuit, and hence that of the entire amplifier stage,'can be adjusted'in. a very simple'manner v and` within wide limits.` The variation in the plate current may beeffected by varying the negativev control grid bias potential, or by varying the screen gridy potential, or by varying simultaneously both Values.

The above mentioned manner of controlling the band width has the advantage thatthe degree of amplification will be influenced to a comparatively, slight. degree-o nly..y For the degree of amplication V of a tube the following equation exists, as isfknuwn: f v l RWE," 40

, Riz-FRI' I Not only doesthe inner resistance R1 depend on the plate current Ja, but also the mutual conductance Sv, and for the normal Ua3/2-charaeteris- 45 tic, Sza-Jal/, wherein a is the constant of the steepness depending only on the construction of the electrode system of the tube. From these krelations it can be readily seen that at an increase .in the plate current Ja, the steepness S increases,

ywhile the inner resistance R1 on the contrary decreases. Since however, the steepness S inycreases with-fJa1/3,jwhile at the same time the -inner resistance Rr decreases, being inversely proportional Ato Ja, the degree of amplication V of a tube measured at constant plate voltage Us as function of the plate current Ja, must increase with an increase in Ja, and must attain a maximum at a definite plate current, whose magnitude depends on the outer resistor Ra and on the product (b-Ua) in order again to decrease at a further increase in Ja. The shape of the curve representing the amplification 'as function of the plate current, and the height of the maximum depend very considerably on the shape of the Ue-characteristic (Ug=grid potential) i. e. on the variation of the steepness as function of the plate current, andcan be infinenced to a wide degree by varying the shape of the characteristic (such as for instance in case of an exponential characteristic).

In the drawings, Fig. 1 shows the `curves for the degree of amplification and band widths measured in the case of a pentode, as function ofthe plate current for various plate potentials. Fig. 2 shows a circuit embodying the invention, and Figs. 3 and 4 show modifications of the circuit. In Fig. 1, the band width plotted in any desired units varies very considerably along these curves. At any small plate currents which are below the value appertaining to the maximum of the curve for the degree of amplification, the selectivity of the resonant circuit .connected to the anode is scarcely infiuenced at all by the inner resistance Ri of the tube, since Ri- Ra. In this range the following equation is approximately valid,

the amplification thus increasing approximately proportional to Jal/3. In the proximity of the maximum of V, Ri is' approximately equal to Ra, so that the band width of the resonant circuit, or to be more exact of the complexresistance formed by the resonant circuit 'and the tube, is approximatelydoubled. If the plate current increases to a value which is approximately twice the current value appertaining to the maximum of the curve for the degree of amplification, then the band width is approximately three times its original value, the stated figures being given only approximately for the sake of comparison, as the accurate values can be read from the curves.

In order to vary the band width the following possibilities are available in accordance with the invention:

(l) Plate potential UE. and screen grid potential Us remain constant; control of the plate current J s being effected by means of the control grid bias potential Ug.

' (2)' Plate potential Ua and control grid bias potential Ug remain constant; control of the plate current Ja being attained by means of the screen grid potential Us. f

(3) Plate potential Us is constant; control of the plate current J a by means of the control grid bias potential Ug and by the screen grid potential Us.

(4) Plate current Je remains constant; control by means of the plate potentialUa. (5) Simultaneous control of the` plate poten- A tial, and of the plate current by one of the meas- -ures specified in 1 to 3. YFor the sake of clarification several means of 'control are indicated in the Figs. 2 and 3.

fied is applied to the input circuit Rg, while the amplified voltage is derived from the output circuit Ra connected to the anode. 'Ihere is available a direct potential obtained from a battery or battery eliminator, and applied to the terminals designated by and and which is applied to the voltage divider P. The latter has connected thereto across the variable taps I, 2, 3 and 4 the control grid, the cathode, the screen grid, and the anode. For control in accordance with case 1, the tap means I is to be displaced, while the remaining tap points remain unvaried. In a corresponding manner, for an adjustment according to 2 only the tap means 3 is to be displaced. A vsimultaneous variation of screen grid potential and control grid bias potential (case 3) can be obtained either by displacing the taps l and 3, or only the tap 2. It is clear that for the fo-urth case the tap 4 is to be varied, while the fifth rcase represents a combination of the previously mentioned possibilities. f

,In Fig. 3 the same reference characters appertain to corresponding parts. The number of electrons about a retarding grid (suppressor grid) B is increased in order to suppress secondary emission, said grid being directly connected with the cathode K as is usually done. A control a'ccording to case 4 can be carried out by introduc-y ing a variable resistor W in the plate circuit.` In view of the high inner resistance of the tube the variation of the plate current is negligibly small, so that only the variation in the plate potential will be effective. A control according tocase 5 canbe carried out, for instance, in vthat a fixed ohmic resistor W is inserted in the anode line, having the effect that a Variation of the plate current produced by one of the previously stated means, automatically also varies the plate potential, in a sense suitable for influencing the band width, in that upon an increase of the plate current, the plate potential decreases.

Finally, through special switching means, it can be accomplished that the degree of amplifif cation remains exactly the same during the control of the band width. From Fig. 1 can be seen, for instance, that at a plate current of Jagl ma. there corresponds to a plate potential of1Ua =200 volts exactly the same amplification 'as for the plate potential Ua: 100 volts at a plate current of Jal ma. However, the band widths for these two conditions have the proportion 1:2.

Herefrom it can be `deduced that through simultaneous increase or variation of the platecurrent and plate potential a control of the band width can be obtained at exactly constant degree of amplification. In practice this result may be approximated by varying lthe plate current by means of a series resistor situated in the screen grid circuit, and at the same time varying-the plate potential by means of a variable series resistor in the anode lead-in, the latter variation being in such a sense that with an increase in plate current the plate potential increases. place of a special series resistor, the same end can also be attained by means of a voltage divider resistance in the respective circuits. As

II).v

tests have shown, it is possible to derive plate potential` and screen grid potential from the same voltage source, and to utilize a common series resistor, or a common voltage divider for the control of the plate potential and screen gridpotential. An example of construction is shown in Fig. 4.

yHere the same tube isl used asin Fig. 2 whereby the voltages to be amplifiedare applied to the sitivity to disturbance.

control grid G, and the amplied power is derived from the plate circuit of said tube across the oscillation circuit Ra. The screen grid potential, and direct plate potential are supplied by a voltage sourcel UA common to both electrodes. In the common circuit of these two electrodes, the series resistor W1 is inserted. Furthermore, fixed or variable resistors W2, Wa may be provided to obtain different steady potentials for the anode and screen grid. 'I'he two last-mentioned resistors may sometimes be omitted (which is readily possible especially in pentodes, or other tubes in ywhich the secondary emission is suppressed), so that the steady screen grid potential and steady plate potential are the same. 'I'he blocking condensers Cb, Cb serve in the known manner to short circuit for alternating potentials.

The said measures for varying the selectivity may obviously also be automatically controlled by the wave to be received and amplified. For instance, the selectivity can be rendered a function of the amplitude of the oscillations to be amplified, in the sense that an increase in the amplitude, a decrease in selectivity occurs, and that in the absence of signals, the amplifier has a minimum band Width, and hence the lowest sen- The automatic control may however, also take place in the opposite sense, in that a high selectivity corresponds with a large input amplitude. The detected voltage must of course be polarized in accordance with the desired type of control, i. e. in accordance as to Whether the selectivity should increase or decrease with an increase inr amplitude.

Figure 3 shows thecase in which aside from a fixed negative control grid bias potential Ug, also a resistor W is inserted in the input circuit of tubeA R, and across which the direct current component of the detected current is passed. In this case there appears at the resistor W' a potential drop, namely the control potential Ugr by which the plate current is influenced. In contrast to similar circuits in which the control of the degree mately constant during the control performance in view of the fact that the working pointis placed into the proximity of the maximum of the curve for the degree of the amplification.

What is claimed is:

1. In a receiver system, a tube having at least a cathode, signal grid, output plate and a positive screen electrode between the grid and plate, a source of signal modulated carrier energy coupled between the cathode and said grid, a resonant circuit connected between said plate and cathode, the circuit being tuned to the -frequency of said carrier, means for varying the plate current and plate potential to adjust the magnitude of the tube innerA resistance while maintaining the tube amplification constant and thereby to vary the degree of selectivity of said resonant circuit, and said tube being operated in the proximity of the maximum of the characteristic curve relating tube amplification to tube plate current,

2. In a high frequency signal amplifier `nctwork having a tube provided with a cathode,`sig nal grid, positive screen and output plate, a tuned load circuit connected to the plate, a signal input circuit connected to the grid, and means for simultaneously varying the plate current and plate potential of the tube in the same sense thereby to adjust the degree of selectivity of the load circuit while maintaining the tube amplification substantally constant.

3. In a high frequency signal amplifier network having a tube provided with a cathode, signal grid, positive screen and output plate, a tuned load circuit connected to the plate, a signal input circuit connected to the grid, means for simul- 9 taneo-usly varying the plate current and plate potential of the tube in the same sense thereby to adjust the degree of selectivity of the load circuit while maintaining the tube amplification substantially constant, said varying means comprising a common direct current source for the screen and plate of the tube, and independent adjustable impedances connected between the screen and plate and the common current source.

HORST ROTH'E.

WERNER KLEEN; 

